Saddle shaped coils are a preferred coil structure for achieving homogeneous fields within a volume for which the axial extent thereof is not large compared to its radial extension. This form is often favored for the cramped environs of an NMR spectrometer probe. The design of such coils and their limitations are discussed in, for example, Hoult and Richards, Journal of Magnetic Resonance, v. 24, pp 71-85, 1976 and Hoult, "Sensitivity Optimization" in Experimental Techniques in .sup.13 C Spectrometry. As with all materials present in the sensitive volume of an NMR apparatus, the inherent magnetic properties of the coil constitute a perturbation of the magnetic field distribution within the volume.
One factor contributing a limitation on the achievable interior field uniformity arises from the finite magnetic susceptibility of the material forming the coils; the conductor, support forms, adhesives and the like. Measures intended to reduce inhomogeneities from this type of perturbation are discussed by Anderson, et al., U.S. Pat. No. 3,091,732, where it was sought to provide coil materials and bonding agents for securing the coil to a coil form, which materials were required to exhibit an effective gross magnetic susceptibility approximating air in which these components are necessarily immersed. Reduction of the magnetic susceptibility to zero (or near zero) through the teaching of U.S. Pat. No. 3,091,732 involves formation of a composite conductor of materials exhibiting dissimilar magnetic properties. The example of a wire comprising a platinum core within a copper sheath requires unusual control in fabrication due to the effects of variation in thickness and position of the platinum core. Alternative fabrication techniques for composites require equivalent close manufacturing control, difficult to achieve or subject to reduced yield in acceptable product. Further developments in manipulating the gross effective magnetic susceptibility of structural members are discussed by Zens, U.S. Ser. No. (482,344).
In addition to perturbations owing to finite magnetic susceptibility, there are perturbations which arise from symmetry properties of the same RF coil structure exhibiting the aforesaid finite magnetic susceptibility. High resolution nuclear magnetic resonance spectrometers routinely employ sample spinning to average over static magnetic inhomogeneities of the polarizing field. Materials exhibiting a susceptibility .chi..sub.o, which are not uniformly distributed azimuthally about the sensitive volume are then averaged to yield a reduced effective magnetic susceptibility .chi.=.chi..sub.o f(.theta.,t), equivalent to smearing the magnetic properties of such materials (especially the coil conductive materials) over a circumferential surface which, however, has no effect upon the axial distribution thereof. It is the axial distribution which remains inhomogeneous.
It is known in the deflection coil art to construct saddle coils with dimensions of component conducting portions which exhibit variation in width and length to distribute the magnetic field radially and axially according to a selected trigonometric function of the deflection angle of a charged particle beam deflected by such angle from the coil axis. Such a system is shown in U.S. Pat. No. 2,830,212.
Accordingly, it is an object of the present invention to geometrically compensate for finite magnetic properties of discrete conductive structures of an RF probe of a high resolution RF spectrometer.